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website: Clarify consistency in FAQ

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Armon Dadgar 2015-03-09 11:46:59 -07:00
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website/source/docs/faq.html.markdown
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@ -50,3 +50,20 @@ TCP and UDP unicast. Broadcast and Multicast are rarely available in a multi-ten
or cloud network environment. For that reason, Consul and Serf were both
designed to avoid any dependence on those capabilities.
## Q: Is Consul eventually or strongly consistent?
Consul has two important subsystems, the service catalog and the gossip protocol.
The service catalog stores all the nodes, service instances, health check data,
ACLs, and Key/Value information. It is strongly consistent, and replicated
using the [consensus protocol](/docs/internals/consensus.html).
The [gossip protocol](/docs/internals/gossip.html) is used to track which
nodes are part of the cluster and to detect a node or agent failure. This information
is eventually consistent by nature. When the servers detects a change in membership,
or receive a health update, they update the service catalog appropriately.
Because of this split, the answer to the question is subtle. Almost all client APIs
interact with the service catalog and are strongly consistent. Updates to the
catalog may come via the gossip protocol which is eventually consistent, meaning
the current state of the catalog can lag behind until the state is reconciled.